I had the same thing at my 20 week u/s. They found 2 cysts. I did a follow up ultrasound 6 weeks later and they were gone. It was a very stressful wait!
My midwife told me the same thing about a marker for down syndrome, but said it was not uncommon for babies to develop cysts on their brains while the brain develops. Its just not always caught on ultrasound. Did you do the quad screen test a few weeks ago? I did it and it was negative which helped ease stress a bit. They also told me the thickness of the spine by the neck was normal as well at the 20 week u/s. I hope for the best for your baby! Hang in there, I know it is so stressful!
I had the exact same thing at my 18 week ultrasound and got sent to a specialist for a more detailed ultrasound. At the specialist the cyst was no longer there and she told me they come and go all the time and are usually not anything to worry about. It's only a concern if they find other physiological abnormalities. At least that's what I was told.
I have not had that experience but I have had friends who were told worrisome things that were seen on ultrasounds that turned out to be false. It happens frequently. I hope it's the same for you!